This invention relates to a method and apparatus for testing the quality of a picture displayed on a color cathode-ray tube, and more particularly to a method and apparatus suitable for automated evaluation of white uniformity of a white display.
Many items are now tested after assembling of color cathode-ray tubes and also after assembling of television sets. The testing of these items is broadly classified into testing of electrical characteristics and testing of picture quality. Automation of the process for testing the electrical characteristics is not so difficult and has already been increasingly put into practice at the site of manufacture of the color picture tubes. On the other hand, the testing of the picture quality is based on a visual inspection of an inspector who looks at a displayed picture to evaluate the quality of the picture. Thus, it is the present status of the art that the visual inspection by an inspector for evaluation of the picture quality is resorted to even in a modern manufacturing line in which almost all of the manufacturing steps have been automated. Therefore, automation of the visual inspection for evaluation of the picture quality is strongly demanded so as to decrease the man-hours required for the visual inspection and to establish a physical and quantitative testing method for evaluation thereby improving the accuracy and reliability of the picture quality inspection.
Among various items of such a visual inspection, evaluation of white uniformity is an item in which the subjective point of view of the inspector is especially strongly manifested. In a practical method for testing the picture quality, a white raster signal generated from a television signal generator is applied to a color cathode-ray tube to be tested, thereby displaying a white picture on the display screen of the color cathode-ray tube. The displayed white picture must ideally be uniformly white throughout the display screen. However, light-colored portions, which are not perfectly white, appear in the displayed white picture, because phosphor dots generating somewhat abnormal luminescence are still present even after the process of adjustment of convergence and landing of electron beams. Such a phenomenon is called "color nonuniformity" herein. An inspector evaluates the degree of this color nonuniformity while observing the white picture and expresses the degree of color nonuniformity in terms of a numerical value called the "white uniformity grade" or briefly "grade". For example, the larger the numerical value, the higher the degree of color nonuniformity.
The color of nonuniform color portions as described above is very light. The result of observation of the color nonuniformity differs depending on the difference between individual inspectors, physical conditions of inspectors, etc. Further, the nonuniform color portions described above appear in various modes. For example, nonuniform color portions having a great variety of tones, color vividness, shapes and sizes appear in the white picture at a variety of positions, and the number of the nonuniform color portions is very large. Thus, the subjective element occupies a greater proportion of the visual evaluation of the white uniformity when compared to the other items of the visual inspection.
Several attempts were made in the past for the purpose of automation of the evaluation of the white uniformity. For example, JP-A-59-75766 discloses a method for quantitative evaluation of the white uniformity in which an image of a picture displayed on a color cathode-ray tube is derived by the use of three primary color filters and a black-and-white television camera and is then binarized, so as to decide whether or not the ratio between the numbers of white and black picture elements is uniform throughout the picture displayed on the color cathode-ray tube.
However, in the prior art method disclosed in the publication cited above, a monochromatic screen such as red, green or blue is used independently, and the physical state of luminescence distribution on the display screen of the color cathode-ray tube is digitized to decide whether the white uniformity is good or bad. Thus, the prior art method was not intended to evaluate the grade of color nonuniformity by extracting the portions exhibiting the color nonuniformity on the white screen where the non-white color portion exists.